Add a short_displayname property to AssignmentGroup

[espenak]

We often need to represent assignmentgroup in listings, breadcrumbs, etc. For now we have solved this with one of the following:

• get_candidates()
• get_students() with an if checking for anonymous assignments.
• A for loop over candidates.
• Sometimes mixing in the name with the above.

This is not good DRY, so we should generalize this in a short_displayname property with the following algorithm::

if anoymous:
    return get_candidates()
elif name:
    return name
else:
    return get_students()

Note that get_candidates and get_students return a list of candidateID/usernames.